1. Field of the Invention
The present invention relates to a scintillator device capable of picking up an image at high sensitivity and high precision, an image pickup apparatus using the scintillator, and a scanning type electron microscope (SEM) for inspection using the scintillator.
2. Description of the Related Art
An example of the image pickup apparatus using a scintillator is an electron microscope camera as disclosed in "Ultramicroscopy", Vol. 52, pp. 7-20 (1993). With this image pickup apparatus, an electron beam image is converted into an optical image by the scintillator, and this optical image is focused on an image pickup element (in this paper, a charge-coupled device (hereinafter abbreviated as CCD), and in others an image pickup tube) by using an optical element (in this paper, an optical fiber plate, and in others an optical lens) to take the object image.
An example of a conventional scintillator whose plates are applied with a voltage is a cathode ray tube type scintillator disclosed in JP-A-58-206029. According to this invention, a photoconductive film and a metal reflection film are formed on the electron beam input plane, and a transparent electrode film is formed on the scintillation output plane. A voltage about 10 V is applied between the metal reflection film and transparent electrode film. As an electron beam enters the scintillator, scintillation occurs due to the cathode luminescence phenomenon. This scintillation emits to the outside via the transparent electrode, and a fraction thereof enters the photoconductive film which has the characteristics of lowering its resistance by light incidence. Therefore, almost all the applied voltage between the metal reflection film and transparent electrode film is applied to the scintillator when light enters. Accordingly, the electroluminescence phenomenon occurs in the local region of the scintillator where electron beams entered, and the scintillator in this region generates light in addition. Namely, light generated by the cathode luminescence phenomenon is superposed upon light generated by the electroluminescence phenomenon so that generated light has a high brightness as compared to light generated only by the scintillator.
A charged particle beam containing image information is scattered in the scintillator by electromagnetic force of atoms having mainly positive charges so that the light generating area has some broad area about the incident axis of the charged particle beam. As a result, even if an image has a sufficient resolution before it enters the scintillator, the image becomes so-called out-of-focus or unsharp when it is converted into an optical image in the scintillator. The electron beam completely loses its image information because of great scattering forms random noises of the image and lowers an S/N ratio greatly.
In order to prevent the resolution and S/N ratio from being lowered, the charged particle beam is arranged to be output from the scintillator before it shifts largely from the incident axis. For example, as disclosed in "Ultramicroscopy", Vol. 54, pp. 293-300 (1994), the scintillator is made thin enough so that the spread width of an electron beam can be suppressed to some degree. This method, however, is associated with the following problems. An electron beam is subjected to electrolytic dissociation in the scintillator and electrons themselves gradually lose their energy while imparting the energy to the scintillator. The scintillator generates scintillation by receiving the energy. The amount of imparted energy increases in proportion to the transmission length of the electron beam in the scintillator. Therefore, if the scintillator is made thin in order to prevent the resolution from being lowered, the amount of scintillation is reduced correspondingly.
The prior technology (JP-A-58-206029) does not aim to control the electron beam direction so that the comparison with this invention is not proper from the point of view of its essence. However the following problems will be explained in terms of the direction control of an electron beam.
For the conventional technology of applying a voltage between the electrodes sandwiching the photoconductive film and scintillator, the photoconductive film is essential. However, if the photoconductive film is used, a large number of electron beams are absorbed in this photoconductive film so that the amount of scintillation is lowered and the electron beam is scattered greatly to present a serious issue of lowering the resolution. The voltage applied between the electrodes is divided by the scintillator and the photoconductive film and the voltage is not applied efficiently to the scintillator, being unable to provide the advantageous effects of electron beam direction control which is the object of this invention. In the conventional applications, in order to generate electroluminescence, an a.c. voltage is required or an alternative application of a d.c. voltage and a zero voltage is required. Only the d.c. voltage cannot generate electroluminescence. However, if the applied voltage becomes zero, the effects of direction control of an electron beam are completely eliminated, and in addition in the case of the a.c. voltage, the electron beam is spread when the polarity is reversed, lowering the resolution. Furthermore, although the applied voltage is in the order of as high as about 10 V, this voltage is too low for the direction control of a charged particle beam which is the object of the invention. For example, a typical acceleration voltage of an electron beam for an electron microscope is 100 kV to 300 kV, and the voltage of about 10 V is impossible to control the electron beam direction.